For the past forty years, tens of thousands of Moroccan soldiers have manned a wall of sand that curls for one and a half thousand miles through the howling Sahara. The vast plain around it is empty and flat, interrupted only by occasional horseshoe dunes that traverse it. But the Berm, as the wall is known, is no natural phenomenon. It was built by the Kingdom of Morocco, in the nineteen-eighties, and it’s the longest defensive fortification in use today—and the second-longest ever, after China’s Great Wall. The crude barrier, surrounded by land mines, electric fences, and barbed wire, partitions a wind-blasted chunk of desert the size of Colorado known as the Western Sahara. Formerly a Spanish colony, the territory was annexed by its northern neighbor, Morocco, in 1975. An indigenous Sahrawi rebel group, called the Polisario Front, waged a guerrilla war for independence. In 1991, after sixteen years of conflict, the two sides agreed to a ceasefire. The wall keeping the foes apart stretches from the Atlantic Ocean to the mountains of Morocco, roughly the distance from New York City to Dallas.

Late last year, I visited the Berm from the Polisario side, to the east, accompanying a handful of supporters of the group from around the world. Until we were about a hundred feet away, I didn’t sense that I was anywhere in particular in the expanse of the desert. My Polisario guide pointed out painted rocks indicating a minefield ahead. A few feet away, an unexploded mortar shell lay in the sand. We walked into a United Nations-controlled buffer zone and the Berm appeared in front of us, rising about six and a half feet behind a barbed-wire fence. I glanced left and right. The wall seemed to stretch endlessly, almost into the blue sky.

As we approached one of the tent-topped fortifications that dot the length of the Berm, a handful of Moroccan soldiers began to scurry around inside. “Will they shoot?” I asked one the Polisario guides. We could see the peaks of the soldier’s flat caps. “No, no,” he replied, laughing. He said that Sahrawis often demonstrate in front of the wall, demanding that Morocco leave the territory. “They’re used to this.” Two women began to shout abuse at the soldiers about Morocco’s King Mohammed VI. “Mohammed, you asshole,” they screamed. “The Sahara is not yours.”

Today, Morocco controls the western eighty per cent of the disputed territory, and the Polisario (which stands for the Popular Front for the Liberation of Saguia el-Hamra and Río de Oro) occupies the rest. The Polisario movement initially began as an armed rebellion against Spanish occupiers. Today, the Polisario calls Western Sahara “Africa’s Last Colony,” asserts that Morocco has replaced Spain as colonizer, and accuses the kingdom of exploiting the territory’s resources. Negotiations have repeatedly stalled, making Western Sahara the site of one of the world’s oldest frozen conflicts. The Polisario’s self-declared Sahrawi Arab Democratic Republic is recognized by the African Union and Algeria, which has given the group military support for decades and currently hosts more than a hundred and seventy thousand Sahrawi refugees in squalid camps.

A Polisario Front official surveys the Moroccan Berm in the Western Sahara. John Bolton and a former German President have helped spur the first negotiations over the disputed desert territory in six years.